The present invention relates generally to apparatus used to dispense a plurality of liquids through a single orifice and, more particularly, to the valving rod that is used to control the flow of the plural liquid components into the mixing chamber via the infeed ports.
Polyurethane foams are formed by the reaction of an isocyanate component and hydroxyl-bearing compounds. When mixed in the presence of a catalyst and other additives, such as a polyether resin, a surfactant, a catalyst, and a blowing agent, these chemicals react to form cross-linked polymer chains, more commonly known as a polyurethane. Each of these components of the plural component material, by itself, is generally stable. Thus, each component will not cure or cross-link for extended periods of time, often as long as several months, if they are properly stored. However, when the isocyanate component and the chemical polyol component, a preformulated compound formed from the aforementioned resin, surfactant, catalyst and blowing agent, are mixed together in proper concentrations, an exothermic chemical reaction of the isocyanate and polyol occurs. This reaction causes a continued expansion that is evidence of the polymerization and manifests itself as foam which cross-links and cures. The cross-linking and curing usually is substantially completed in a matter of seconds.
Polyurethane foam dispensers are well known and have achieved a high degree of usage in factories where components must be adhesively lined with an insulating foam or where products must be packaged and protected from damage during shipment. This high level of use of polyurethane foam dispensing equipment has also focused attention on the efficiency of prior apparatus employed to accomplish this dispensing. Many of the problems with foam dispensing equipment stem from the fact that the polyurethane foam "sets" or builds up in the dispenser, normally within the mixing chamber or the nozzle, after the chemical components have cross-linked and begun to cure. This "setting" can eventually cause the apparatus to become inoperative due to clogging or blockage of the flow passages.
In operations such as those required during packaging, where intermittent use of the dispensing apparatus is required, the "setting" problem is more severe. This typically occurs in the situation where a packer initially directs a "shot" of the mixed plural components into the bottom of a container, inserts a polyethylene strip over the top, and places the product to be shipped in the container. Another sheet of polyethylene is placed on top of the product, and the foam dispensing apparatus is then activated after a delay of 10 to 20 seconds from the time the first "shot" was dispensed to fill the box with the cushioning foam. This procedure is repeated for each item to be packed.
Prior dispensing equipment have attempted to solve this "setting" problem by using either separately or combinatively air blasts, cleaning rods or plungers with scrapers, or solvent to remove the residue foam from the dispensing assembly. Specific polyurethane foam systems have attempted to use air with pressurized solvent blown into the mixing chamber and the dispensing nozzle, and automatic solvent flush that runs through the mixing chamber and dispensing nozzle, a blast of purging gas preceding the continuous pumping of solvent through the mixing chamber and dispensing nozzle, and reciprocating cleaning rods or plungers which may or may not use the solvent to faciliate the scraping for residue foam from the dispensing apparatus.
The use of a combination cleaning rod and valving rod to control the flow of the polyol and isocyanate components has proven to be the most common design employed in commercial applications. To be successful, however, the length of the valving rod must be within closely controlled tolerances so that in its extended position the rod extends out the end of the mixing chamber to thoroughly clean any of the residue foam from the end and, in its retracted position, it retracts to the rear of the inlet orifices that supply the isocyantae and polyol components. A valving rod that is too short will not remove all of the foam from the end of the mixing chamber and clogging will eventually occur. A valving rod that is too long will not permit the inlet orifices to be uncovered in the retracted position to have the isocyanate and polyol components enter the mixing chamber with unrestricted flow, or at all. Additionally, a valving rod that is too long permits the foam to adhere to the end of the rod and then will score the inner plastic portions of the mixing chamber upon retraction.
With the advent of removable mixing chambers and dispensing nozzles, disassembly of the dispensing apparatus to include removal of the valving rod is a normal part of the maintenance operation. The need to be able to remove the valving rod easily and replace it easily with the same length as prior to disassembly has taken on increased importance.
These problems are solved in the design of the present invention by providing an adjustably positionable valving rod to ensure the proper rod length is obtained and to permit the rod to be easily removed from a plural component dispensing assembly.